This application claims the benefit of Japanese Application P2001-101,729, filed Mar. 30, 2001, and Japanese Application P2002-015,167, filed Jan. 24, 2002, the entireties of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to optical waveguide devices and travelling-wave optical modulators utilizing the same.
2. Related Art Statement
A traveling-wave optical modulator using lithium niobate (LiNbO3), lithium tantalate (LiTaO3) or gallium-arsenide (GaAs) for the optical waveguide has excellent properties and may realize a broadband modulation at a high efficiency. Lithium niobate and lithium tantalate are excellent ferroelectric materials having large electro-optic coefficients and can control light within a short optical path. Factors suppressing the modulation frequency of the traveling-wave optical modulator include velocity mismatch, dispersion, electrode conductor loss and dielectric loss.
The concept of velocity mismatch will now be further explained. In a traveling-wave optical modulator, the velocity of the light propagating through the optical waveguide largely differs from that of the signal microwave propagating through the electrode. Assume that the light and the modulation wave propagating through the crystal have different velocity Vo and Vm, respectively. For example, calculation was made for an LiNbO3 optical modulator having planar type electrodes. The effective refractive index of LiNbO3 single crystal is 2.15, and the velocity of the light propagating through the optical waveguide is inversely proportional to the refractive index. On the other hand, the effective index for modulating wave is given by a square root of the dielectric constant near a conductor propagating the wave. LiNbO3 is a uniaxial crystal, and the dielectric constant in the Z-axis direction is 28 and those in the X-axis and Y-axis directions are 43. Therefore, even if an influence of air having a dielectric constant of 1, the effective index microwave in the LiNbO3 optical modulator having a conventional structure is about 4, which is about 1.9 times 2.15. Thus, the velocity of the light wave is about 1.9 times as much as that of the modulating microwave.
The upper limit of bandwidth xe2x80x9cfmxe2x80x9d of the optical modulator or the modulating velocity is inversely proportional to a difference in velocity between the light wave and the modulating wave. That is, fm=1/(Voxe2x88x92Vm). Therefore, assuming that the power loss by electrode is zero, the upper limit of bandwidth xe2x80x9cfmxe2x80x9d x electrode length xe2x80x9c1xe2x80x9d=9.2 GHz.cm. Actually, it has been reported that in an optical modulator having an electrode length xe2x80x9c1xe2x80x9d=2.5 mm, fm=40 GHz. The effect due to the upper limit of modulation speed becomes more substantial as the electrodes become longer. Therefore, an optical modulator having a broadband modulation and low driving voltage has been earnestly demanded.
The inventors have considered the following idea. That is, the velocity matching between signal microwave and light wave may be realized by applying a thin plate with a thickness of, for example, 10 xcexcm for an optical waveguide substrate. It is, however, difficult to obtain a plate with such a small and uniform thickness by grinding. The resulting plate has a low strength and may easily be warped so that it may be useless.
The assignee filed a Japanese patent laid-open number 133, 159/1998 (U.S. Pat. No. 6,219,469), and disclosed a travelling wave optical modulator for giving the solution. The modulator has an optical waveguide substrate having a thinner portion with a thickness of not more than 10 xcexcm where the optical waveguide is formed. It is thereby possible to realize high-speed modulation without forming a buffer layer made of silicon dioxide, and to advantageously reduce the product xe2x80x9cVxcfx80xc2x7Lxe2x80x9d of a driving voltage Vxcfx80 and a length of an electrode xe2x80x9cLxe2x80x9d.
The inventors have studied the whole process for manufacturing a travelling wave optical modulator. They have tried to form a recess on the surface of an optical waveguide substrate by machining, as described in the Japanese Patent 133, 159/1998, to form a thinner portion with a thickness of, for example, not more than 10 xcexcm. They found the following problems. FIG. 10 schematically shows such substrate 16. A deep recess 17 is formed by, for example laser beam working or grinding, from the back face 16b of the substrate 16. The substrate 16 has a thickness of, for example, 0.3 mm and the thinner portion 16c has a thickness of, for example, 10 xcexcm. A thicker portion 16a remains after the working in the substrate 16 to preserve the mechanical strength. 16d is a worked surface.
In an actual working process, however, it may be difficult to form the recess with an ideal shape shown in FIG. 10. For example, the recess is formed by laser beam working using a lens. As the recess 17 is deeper, the focus of the lens moves so that the worked surface 16d is curved or rounded. It is therefore difficult to maintain the thickness of the thinner portion 16c at a specific value. The thickness of the central part of the thinner portion 16c tends to be considerably smaller than that of the peripheral parts of the thinner portion. As a result, when the working is performed so that the thickness of the thinner portion 16c is maintained not larger than a specified value, for example not larger than 10 xcexcm, over a sufficiently wide area, the thickness of the central part of the thinner portion 16c becomes considerably smaller than 10 xcexcm. In other words, if the thickness of the peripheral part of the thinner portion 16c is adjusted to 10 xcexcm, the central part of the thinner portion 16c is made considerably smaller than 10 xcexcm. Such thin central part with a thickness of smaller than 10 xcexcm may easily be broken as 16e. When the substrate 16 is worked using a grinding stone, the above problems may not be avoided.
An object of the invention is to provide an optical waveguide device having an optical waveguide substrate having a mechanical strength sufficient for handling, being effective for reducing off-specification products due to warping, cracks and fracture in the substrate, and effective for improving the propagating velocity of signal wave applied onto its electrode.
Another object of the invention is to apply the above optical waveguide device to a travelling-wave optical modulator to improve the velocity matching of signal wave applied onto its electrode and light wave propagating through the optical waveguide.
The invention provides an optical waveguide device comprising an optical waveguide substrate and a supporting substrate supporting the optical waveguide substrate wherein the optical waveguide substrate comprises a main body made of an electro-optic material and having a first main face and a second main face, an optical waveguide formed in or on the main body and an electrode formed on the side of the first main face of the main body. The supporting substrate is joined with the second main face of the main body, and a low dielectric portion with a dielectric constant lower than that of the electro-optic material is formed in the supporting substrate.
The invention further provides a travelling-wave optical modulator having the device, wherein a voltage for modulating the light propagating through the optical waveguide is applied by means of the electrode.
The inventors studied the problems described above, and have reached the conclusion that the problems might not be totally avoided in a process of forming the thicker portion 16a for improving the strength and an space 17 for improving the propagating velocity of microwave signal in the substrate 16, as shown in FIG. 10. The inventors have tried to ensure a sufficient strength of a device by providing a separate supporting substrate. That is, the supporting substrate is joined with an optical waveguide substrate to provide a mechanical strength sufficient for handling the device. As a result, it is not necessary to provide the thicker portion in the optical waveguide substrate for assuring the mechanical strength sufficient for handling, so that the total thickness of the optical waveguide substrate may be considerably reduced. The inventors also have reached the idea that a recess or space is formed in the supporting substrate to utilize the recess or space for improving the propagating velocity of microwave in the electrode.
It is therefore possible to provide a mechanical strength sufficient for handling an optical waveguide device as a whole and to prevent the warping of its optical waveguide substrate at the same time. Moreover, it is possible to reduce off-specification products due to cracks or fractures caused during the working process for forming a thinner portion in the optical waveguide substrate. It is thereby possible to reduce the thickness of the optical waveguide substrate and to improve the propagating velocity of signal wave applied onto the electrode by providing a space in the supporting substrate. Any kinds of low dielectric portions with a relatively low dielectric constants may be used instead of the recess or space.
The invention further provides an optical waveguide device comprising an optical waveguide substrate and a supporting substrate supporting the optical waveguide substrate wherein the optical waveguide substrate comprises a main body made of an electro-optic material and having a first main face and a second main face, an optical waveguide formed in or on the main body and an electrode formed on the side of the first main face of the main body. The device further comprises a joining layer joining the supporting substrate and the second main face of the main body and a low dielectric portion with a dielectric constant lower than that of the electro-optic material. The low dielectric portion is provided inside the joining layer between the main body and the supporting substrate.
The inventors further found that the above effects may be obtained by providing a low dielectric portion inside of the joining layer, instead of providing the low dielectric portion in the supporting substrate.
These and other objects, features and advantages of the invention will be appreciated upon reading the following description of the invention when taken in conjunction with the attached drawings, with the understanding that some modifications, variations and changes of the same could be easily made by the skilled person in the art.